The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the quantitative measurement of cerebral perfusion with an MRI system.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Perfusion as related to tissue refers to the exchange of oxygen, water and nutrients between blood and tissue. The measurement of tissue perfusion is important for the functional assessment of organ health. Perfusion weighted images (PWI) show the degree to which tissues are perfused by the change in their brightness as a bolus of contrast agent washes through the vasculature, and can be used to assess the health of brain tissues that have been damaged by a stroke. A number of methods have been used to produce perfusion images using magnetic resonance imaging techniques. One technique, as exemplified by U.S. Pat. No. 6,295,465, is to determine the wash-in or wash-out kinetics of contrast agents such as chelated gadolinium. From the acquired NMR data, images are produced which indicate cerebral blood flow (CBF), cerebral blood volume (CBV), and mean transit time (MTT) at each voxel. Each of these perfusion indication measurements provides information that is useful in diagnosing tissue health.
Bolus tracking cerebral perfusion has expansive use in the clinical setting for imaging a variety of diseases including cerebrovascular occlusive disease, stroke, central nervous system tumors, and Alzheimer's disease. Parametric images of cerebral perfusion are calculated by analyzing the tracer kinetics of a known contrast agent, whether it is radio-labeled water in positron emission tomography (PET), an iodinated contrast agent in computed tomography (CT), spin-labeled water in arterial spin labeling MRI, or a paramagnetic contrast agent in dynamic susceptibility contrast (DSC) MRI. While the standard for quantification of cerebral perfusion still remains radio-labeled PET imaging, the requirement of a cyclotron for production of the radio-labeled tracer limits the availability of the technique. CT has the potential to quantify perfusion; however, iodinated contrast agents and large doses of radiation are required in this imaging method. This is problematic for frequent follow-up scan session as well as the use of the method in certain patient populations, such as young children.
MR-based perfusion imaging methods produce parametric images that only convey information relating to relative, and not quantitative, cerebral blood flow (rCBF) and cerebral blood volume (rCBV). Current methods for creating quantitative measurements of perfusion from MR imaging data rely on assuming population averaged values of normal appearing white matter (WM) and by setting the CBF values in this tissue to a preset value. This method has a poor correlation to PET imaging standards. Instead, a method which determines the quantitative CBF and CBV (qCBF and qCBV, respectively) on a subject-by-subject basis would be preferred.